Q:
I still don't understand the fact that all that photos show
something expanding in a "circular/planar" situation and not in
a "spherical/3D" configuration. Could that be that the circular
expansion is a concentration of a spherical explosion seeing
sideways--a mere optical effect?

A:
Yes, many of the 2D "rings" are actually 3D expansions that
appear as rings because of a phenomenon known as "limb
brightening". If the expanding material is in a 3D shell, the
telescope will look through more material on the edges of the
shell than the center, causing it to appear as a bright
ring.